1. Field of the Invention
The present invention relates to the field of optical scientific instrumentation. More specifically, the present invention relates to the dynamic velocity control of a moving beamsplitter and one or more of the reflective components utilized in a Fourier-Transform infrared (FTIR) interferometer.
2. Discussion of the Related Art
An optical interferometer used in a scientific analytical instrument relies on the interference of superimposed optical beams as part of the interrogation means. When configured as a Michelson Fourier-Transformed infrared (FTIR) instrument, the optical output of the interferometer is called an interferogram. The FTIR interferometer itself often includes a beamsplitter and two mirrors, one that is conventionally stationary, and one which is conventionally mobile. The mobile mirror moves along the optic axis while staying optically perpendicular to the light beam at all times. The movement of the mobile mirror is often desired to be feedback controlled in order to hold the mirror velocity constant so that the analytical radiation that passes through the interferometer produces an accurate interferogram. Conventional interferometers have a moving mirror assembly that includes a linear ball bearing, air bearing, slide bearing, or a flexure bearing and is often driven by a linear motor (e.g., a coil coupled to a permanent magnet) to provide velocity control.
Motion and a resultant velocity of the mobile mirror in a conventional system can be tracked by a positioning monochromatic beam of optical radiation operating in conjunction with the analytical radiation beam passing through the interferometer optics. The configured monochromatic beam (e.g., laser) is thus also often partially reflected and partially transmitted through the configured beamsplitter of the interferometer, and because of the design, the split beams are reflected from the conventionally fixed mirror and the conventionally mobile mirror and recombined at the beamsplitter.
The recombined beams at the beamsplitter are thereafter directed to a detection means that can thereby determine the tilt, position, and/or velocity of the mobile mirror along its longitudinal translation axis. With respect to tilt, the deviations in the phases of the components of the recombined beams can be used to indicate a misalignment of the mobile mirror with respect to a perpendicular of the designed longitudinal axis for the optical element. If such deviations are deleterious, a tilt servo controller can apply corrective forces to the support of the mobile mirror so as to realign the face of the mirror. With respect to the position/speed of the mobile mirror, a velocity control servo can analyze the fringe information of the combined beams so as to compare the determined velocity to a desired velocity and generate an additional correction force so as to move the mirror in a controlled linear fashion.
Background information on such an interferometer system that utilizes dynamic velocity and tilt control of the moving mirror, is described and claimed in, U.S. Pat. No. 5,883,712, entitled, “INTERFEROMETER OF AN INFRARED SPECTROMETER WITH DYNAMIC MOVING MIRROR ALIGNMENT” issued Mar. 16, 1999, to John M. Coffin, including the following, “[i]n accordance with the present invention, an interferometer for an infrared spectrometer provides dynamic alignment of the moving mirror to maintain precise alignment between the moving mirror and the fixed mirror. The alignment of the moving mirror in this manner maximizes the stability of the interferometer while achieving high levels of output accuracy despite vibrations due to the movement of the moving mirror on its bearings and vibrations transmitted from external sources to the interferometer. The dynamics of the mounting of the moving mirror allow the position of the mirror to be controlled with high accuracy even in the presence of relatively high frequency vibrations. The structure of the interferometer and of the detectors and controls for maintaining the alignment of the moving mirror are nonetheless simple in construction and contribute relatively little additional bulk or weight to the interferometer.”
However, while current dynamic corrective measures, such as, but not limited to, those described above in U.S. Pat. No. 5,883,712 are beneficial, the performance is often limited by all the delays in the configured control loops, the result of which are degraded response times. Moreover, rolling and sliding bearings used in conventional systems so as to move the mobile mirror add velocity error noise as the non smooth parts slide or roll past one another. In addition, because only one of the optical elements is mobile as compared to a plurality of parts moving, as described by the present invention, the response time is inherently longer and the control errors are increased, thus leading to increased errors in the spectral data created by the analytical instrument.
Accordingly, the present invention addresses the need for an improved dynamic velocity control system as utilized in scientific optical interferometers, such as, a Fourier Transform infrared (FTIR) interferometer, by incorporating among other aspects, the novel moving beamsplitter and moving mirror designs presented herein.